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I I I I I I I I I I I I I I I I I I I Obstructions to flood flows within the study reach include natural obstructions such as large rocks, boulders, and brush and other vegetation growing along the streambank as well as man-made obstructions such as bridges, buildings and culverts. During floods, these obstructi ons impede flood flows and cause backwater condi ti ons that may increase the flood hei ghts upstream of the obstructi on and vel oci ti es downstream of the obstruction. Brush and rock washed out during floods is carri ed downstream and may coll ect at bri dges and in cul verts, thus creati ng a darrmi ng effect and overbank flow. As flood fl ows increase, masses of debris may break loose and cause a wall of water and debris to surge downstream until another obstruction is encountered. In some instances, debris may collect to the point where structural capability is exceeded and a bridge is destroyed or abutments and roadbeds damaged. HISTORY OF FLOODING Large floods in the basins were reported in 1864, 1876, 1894, 1914, 1923, 1938, 1941, 1956 and 1969. Descriptions of the major floods that have affected either the Upper Boulder and Fourmile Creek basins are presented below. A detai 1 ed flood hi story of the county, prepared by Bou1 der County, will be published in the near future. That publication will contain more detailed information than given here. Flood of May 23-23, 1876 - A general storm over the Boulder Creek basin created flooding on the plains of reportedly 1 1/2 miles wide. Railroad service to Boulder was disrupted. Flood of May 29-June 2, 1894 - Heavy rains fell over the mountains extendi ng from the Co1 orado-Hyomi ng border south~lard into the Repub1 i can and Arkansas River basins. Rainfall over the Boulder and South Boulder Creek basins was particularly heavy. Rainfall records for a 96-hour period ending at 3:00 a.m. on June 2, 1894 indicate that the mountain drainage area received from 4.5 to 6.0 inches of precipitation. Rainfall amounts over the hi 9h p1 ai ns gradually decreased from west to east and varied from 5 inches at Boulder to approximately 2.5 inches at the mouth. The mountain rainfall combined with the snowmelt runoff to produce the greatest flood known at Boulder inundating the valley during the night of May 30, 1894. Buildings, bridges, roads and railroads were washed away. All bri dges were reportedly swept away in Boul der Canyon with the highway and rail road almost totally destroyed as far up the canyon as Fourmile Creek and up Fourmile Creek as far as Sunset. The Boulder Camera stated that six buildings were destroyed at Crisman, eight 4